The strong demand for a reduction in emissions of greenhouse gases, combined with increasingly stringent automobile safety requirements and fuel prices, has forced the builders of motor vehicles to make increasing use of steels that have improved mechanical strength in the bodies of their vehicles to reduce the thickness of the parts and therefore the weight of the vehicles, while maintaining the mechanical strength of the structures. In this regard, steels that combine high strength with sufficient formability without the appearance of cracks are acquiring increasing importance. Several families of steels that offer different levels of mechanical strength have been proposed in the past, one after another.
Steels have been proposed that contain micro-alloy elements, the hardening of which is obtained simultaneously by reducing the grain size and by fine precipitates. The development of increasingly hard steels led to the increased use of “dual phase” steels, where the presence of martensite in a ductile ferrite matrix makes it possible to achieve mechanical strength greater than 400 MPa associated with good suitability for cold forming.
To achieve characteristics of mechanical strength, ductility and formability which are even more advantageous for the automobile industry, for example, “TRIP” steels, for “Transformation Induced Plasticity” have been developed. These steels have a complex structure that includes a ductile structure, ferrite as well as martensite, which is a hard structure that contributes the high mechanical characteristics, and residual austenite, which contributes both strength and ductility thanks to the TRIP effect.
This TRIP effect designates a mechanism according to which, under additional deformation, such as during uniaxial stress, for example, the residual austenite of a sheet or blank made of TRIP steel is progressively transformed into martensite, which translates into a significant consolidation which retards the appearance of cracks. Nevertheless, TRIP steels exhibit mechanical strengths of less than 1000 MPa because their content of polygonal ferrite, which is a relatively weak and highly ductile structure, is greater than one-quarter of the total area percentage.
To meet this demand for steels with a mechanical strength greater than 1000 MPa, it is therefore necessary to reduce the structural fraction with low mechanical strength and to replace it with a phase that contributes greater hardening. However, it is known that in the field of carbon steels, an increase in mechanical strength is generally accompanied by a loss of ductility. In addition, the builders of motor vehicles are specifying increasingly complex parts that require steels that make it possible to achieve a bendability greater than or equal to 90° without the occurrence of cracking
The contents of the chemical elements listed below are indicated in per cent by weight.
The relevant prior art also includes WO2007077933, which describes a microstructure composed of bainite, martensite and residual austenite. The chemical composition of the sheet disclosed in '933 includes 0.10-0.60% C, 1.0-3.0% Si, 1-3.5% Mn, up to 0.15% P, up to 0.02% S, up to 1.5% Al and 0.003 to 2% Cr, with the remainder consisting of iron and impurities. The microstructure in the framework of this patent is obtained during the annealing by a hold after the primary cooling at a temperature below the starting point of the martensite transformation Ms. The result is a microstructure which includes a mixture of tempered and/or partitioned martensite. The principal advantage claimed is an improvement in the resistance to hydrogen embrittlement. The presence of martensite, which is a hardening component in a softer bainitic matrix, makes it impossible to achieve the ductility and bendability expected in the framework of the invention.
The relevant prior art also includes GB 2,452,231, which describes the fabrication of steel sheet with a strength greater than 980 MPa with a satisfactory ultimate strength and properties that are satisfactory in terms of hole expansion and spot welding. The chemical composition of the sheet disclosed in '231 includes 0.12-0.25% C, 1.0-3.0% Si, 1.5-3% Mn, up to 0.15% P, up to 0.02% S and up to 0.4% Al, with the remainder consisting of iron and impurities. In addition, the ratio of the content of Si by weight to the content of C by weight, Si/C, is in the range of 7-14. The microstructure of the sheet contains at least 50% bainitic ferrite, at least 3% residual austenite in the form of laths, austenite in solid form, the average grain size of which is less than or equal to 10 micrometers, whereby this solid austenite is present in the amount of 1% up to one-half of the content in terms of austenite in laths. This prior art patent provides no information on the bendability of the sheet produced and mentions the absence of carbides in the bainite.